An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing

We recently reported the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the  Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (T ~ 270 days), high-magnification microlensing event MOA-11-191/OGLE-11-461, in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position.  Combining the photometric, astrometric, and spectroscopic measurements, we obtain a lens mass of 7.1 +/- 1.3 solar mass and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a “natal kick'' from its supernova explosion. Our mass measurement is the first ever for an isolated stellar-mass BH using any technique.